Calibration of optical line shortening measurements

ABSTRACT

A system and method of calibrating optical line shortening measurements, and lithography mask for same. The lithography mask comprises a plurality of gratings, with a calibration marker disposed within each grating. The mask is used to pattern resist on a semiconductor wafer for purposes of measuring and calibrating line shortening. The pattern on the wafer is measured and compared to measurements made of the pattern on the mask. The difference gives the amount of line shortening due to flare, and may be used to calibrate line shortening measurements made using optical measurement tools.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the following and commonly assigned U.S.patent application: Ser. No. 10/964,102, filed Oct. 13, 2004 and issuedas U.S. Pat. No. 7,096,127, entitled “Measuring Flare in SemiconductorLithography,” which application is hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates generally to lithography of semiconductordevices, and more particularly to the calibration of line shorteningmeasurements.

BACKGROUND

Semiconductor devices are manufactured by depositing many differenttypes of material layers over a semiconductor workpiece or wafer, andpatterning the various material layers using lithography. The materiallayers typically comprise thin films of conductive, semiconductive andinsulating materials that are patterned to form integrated circuits.

One type of semiconductor lithography involves placing a patterned maskbetween a semiconductor workpiece, and using an energy source to exposeportions of a resist deposited on the workpiece, transferring the maskpattern to the resist. The resist is then developed, and the resist isused as a mask while exposed regions of a material on the workpiece areetched away.

As semiconductor devices are scaled down in size, lithography becomesmore difficult, because light can function in unexpected andunpredictable ways when directed around small features of a mask.Several phenomenon of light can prevent the exact duplication of a maskpattern onto a wafer, such as diffraction, interference, or flare, asexamples.

In many designs, the individual features of a circuit, such as gatelines or signal lines, as examples, have extremely small dimensions andmay have widths of about 0.1 to 0.4 μm or less, with their lengths beingconsiderably greater than the widths, e.g., about 0.8 to 2.0 μm orgreater, for example. These thin lines may be connected to other layersof the integrated circuit by narrow vias filled with conductivematerial. When dimensions reach such a small size, there is not only atendency for a line formed on a wafer to be shorter than its designlength as defined by the lithography mask, but also the positioning ofthe vias may not be aligned to the target structures. Transferdifferences of such critical dimensions occur when a desired circuitfeature is particularly thin or small because of various opticaleffects. The accuracy of forming and positioning the lines and the viasbecomes increasingly critical as dimensions decrease. Relatively minorerrors in positioning such features can cause a via to miss the linealtogether or to contact the line over a surface area that isinsufficient to provide the necessary conductivity for a fullyfunctional circuit. Thus, it is important to determine the effects ofand to account for line shortening using optical measurements.

In the measurement of line shortening, gratings can be used to determinebest focus and exposure dose, as described in an article entitled,“Focus Characterization Using End of Line Metrology,” by Leroux et al.,in IEEE Transactions on Semiconductor Manufacturing, Vol. 13, No. 3,August 2000, pp. 322-330, and in an article entitled, “DistinguishingDose From Defocus for In-Line Lithography Control,” by Ausschnitt, inProc. SPIE, Vol. 3677, pp. 140-147, which are incorporated herein byreference. The ends of the thin lines of a grating structure aresensitive to focus, yet are “seen” as a solid line by an opticalmetrology tool using white light. The ends of lines appear as a solidline because the wavelength of the light is significantly larger thanthe printed features.

FIG. 1A shows a prior art sub-resolution grating pattern on alithography mask. When imaged on a wafer or workpiece and viewed withwhite light on a wafer, an optical tool interprets the ends of thesub-resolution grating 209 lines as a solid line 207, as shown in FIG.1B. However, when viewed with a scanning electron microscope (SEM), thegratings are visible, and the ends of the sub-resolution grating 209lines appear as a single, somewhat ragged line, as shown in FIG. 1C.Because the lines and spaces of the grating on the mask 208 are smallerthan the wavelength of light used in an optical microscope, the patternformed on the workpiece is not visible by an optical tool, as shown inFIG. 1B. For example, the lines and spaces of the grating pattern 209may be about 0.12 to 0.15 μm, and white light used in an opticalmicroscope to view the workpiece may comprise a wavelength of about 650nm or less, which is too large to resolve the pattern. However, a SEMhas a higher resolution and can detect the pattern formed on theworkpiece, and thus a SEM must be used to resolve the pattern on theworkpiece.

Though an optical alignment tool can measure the relative lineshortening, the measurements are sensitive to pitch and contrast, asdescribed in an article entitled, “Understanding Optical End of LineMetrology,” by Ziger, et al., in Opt. Eng., July 2000, Vol. 39, No. 7,pp. 1951-1957, which is incorporated herein by reference. For certainapplications, a relative measurement is all that is required to measureline shortening. As an example, optimum focus can be determined from theminimum line shortening a modified box-in-box structure, as described inthe article “Focus Characterization Using End of Line Metrology,”previously referenced herein. However, the actual line shortening andline shortening optically measured are quite different due todiffraction effects.

There are applications using gratings that are read with optical tools,for which an absolute value of line shortening is desirable. Forexample, in U.S. Pat. No. 5,962,173, issued on Oct. 9, 1999, entitled,“Method for Measuring the Effectiveness of Optical ProximityCorrections,” which is incorporated herein by reference, Leroux, et al.describe a method for measuring optical proximity effect using amodified box in box structure that contains gratings which are readusing an optical measuring instrument. This method requires calibrationof the optical metrology tool to ensure that the measurements arecorrect.

In U.S. Pat. No. 6,301,008, issued on Oct. 9, 2001, entitled,“Arrangement and Method for Calibrating Optical Line ShorteningMeasurements,” which is also incorporated herein by reference, Ziger etal. describe an approach to calibrate optical and SEM-basedmeasurements. However, this approach requires a correlation betweenactual and optical measurements as a function of pitch and line size.

A SEM cannot be used to make measurements of line shortening, becausethe gratings of the test patterns are too long for the SEM to measure. ASEM is useful for high magnification measurements, but a SEM cannotmeasure the line length of a long line. For example, if there is lineshortening of 0.1 μm in a test pattern grating having a length of 9 μm,a SEM cannot be used to measure the line shortening, and thus a SEMcannot be used to calibrate optical metrology tools for such a testpattern.

What is needed in the art are improved methods of calibrating lineshortening measurements of optical measurement or metrology tools insemiconductor lithography systems.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present invention, which provide improved methods of calibratingline shortening measurements. A lithography mask comprises a gratingstructure, with a calibration marker placed within each grate. Themarker is small enough to be unresolvable by optical wavelengths, andprovides an absolute measurement of line shortening, which can be usedto calibrate optical measurements made by optical metrology tools.

In accordance with a preferred embodiment of the present invention, alithography mask for calibrating line shortening measurements includes-atransparent substrate, and an opaque material disposed on the substrate.The opaque material includes a test pattern, the test pattern comprisinga plurality of gratings, at least one of the gratings comprising acalibration marker disposed therein.

In accordance with another preferred embodiment of the presentinvention, a method of calibrating line shortening measurements includesproviding a semiconductor workpiece, the workpiece comprising a layer ofphotosensitive material disposed thereon, and exposing the layer ofphotosensitive material to energy through at least one mask, the atleast one mask comprising a first test pattern and a second test patterndisposed proximate a portion of the first test pattern, the second testpattern comprising an opaque or attenuated region, the first testpattern comprising a plurality of gratings, wherein a calibration markeris disposed within at least one of the gratings. The method includesdeveloping the layer of photosensitive material, measuring featuresformed on the photosensitive material by the first test patternproximate the second test pattern to determine a first line shorteningmeasurement, and measuring the length of at least one grating of thefirst test pattern of the at least one mask. A first side of the atleast one grating formed by the at least one grating of the first testpattern on the photosensitive material is measured between a calibrationmarker and an end of the grating, and a second side of the at least onegrating formed by the at least one grating of the first test pattern onthe photosensitive material is measured between the calibration markerand the other end of the grating. The method includes determining thedifference D1 of the first side measurement and the mask measurement,determining the difference D2 of the second side measurement and themask measurement, and determining the difference of D1 and D2 todetermine a second line shortening measurement. The second lineshortening measurement is compared to the first line shorteningmeasurement to determine a calibration factor for line shorteningmeasurements.

In accordance with yet another preferred embodiment of the presentinvention, a system for calibration of line shortening measurementsincludes a semiconductor workpiece, the workpiece comprising a layer ofphotosensitive material disposed thereon, and at least one mask, the atleast one mask comprising a first test pattern and a second test patterndisposed proximate a portion of the first test pattern, the second testpattern comprising an opaque or attenuated region, the first testpattern comprising a plurality of gratings, wherein a calibration markeris disposed within at least one of the gratings. The system includes anexposure tool for exposing the layer of photosensitive material toenergy through the at least one mask, and a developing tool fordeveloping the layer of photosensitive material. The system includes atleast one measurement tool for measuring features formed on thephotosensitive material by the first test pattern proximate the secondtest pattern to determine a first line shortening measurement, formeasuring the length of at least one grating of the first test patternof the at least one mask, for measuring a first side of the at least onegrating formed by the at least one grating of the first test pattern onthe photosensitive material between a calibration marker and an end ofthe grating, and for measuring a second side of the at least one gratingformed by the at least one grating of the first test pattern on thephotosensitive material between the calibration marker and the other endof the grating. The system includes a processor adapted to determine thedifference D1 of the first side measurement and the mask measurement,the difference D2 of the second side measurement and the maskmeasurement, the difference of D1 and D2 to determine a second lineshortening measurement, and adapted to compare the second lineshortening measurement to the first line shortening measurement todetermine a calibration factor for line shortening measurements.

Advantages of embodiments of the present invention include providingimproved methods of calibrating line shortening measurements. Anabsolute distance between a calibration marker and the end of a line ismeasured, providing an absolute line shortening measurement. The effectsof diffraction can be segregated, and the effects of flare in lineshortening can be distinguished. The resulting calibration factor can beused to correct or calibrate measurements taken by an optical metrologytool. The method reduces the amount of error in the measurement of lineshortening.

The foregoing has outlined rather broadly the features and technicaladvantages of embodiments of the present invention in order that thedetailed description of the invention that follows may be betterunderstood. Additional features and advantages of embodiments of theinvention will be described hereinafter, which form the subject of theclaims of the invention. It should be appreciated by those skilled inthe art that the conception and specific embodiments disclosed may bereadily utilized as a basis for modifying or designing other structuresor processes for carrying out the same purposes of the presentinvention. It should also be realized by those skilled in the art thatsuch equivalent constructions do not depart from the spirit and scope ofthe invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1A shows a sub-resolution grating pattern formed on a lithographymask;

FIG. 1B shows the grating pattern of FIG. 1A formed on a resist of asemiconductor wafer, as viewed from an optical microscope;

FIG. 1C shows the grating pattern of FIG. 1A when detected by a SEM at alow resolution;

FIG. 2A shows a lithography mask having a flare-sensitive pattern formedthereon which may be used to expose a resist of a semiconductorworkpiece;

FIG. 2B shows a lithography mask having an opaque pattern that ispositioned asymmetrically over a latent image formed on a resist usingthe lithography mask of FIG. 2A in order to measure the effect of flare;

FIG. 3 shows a lithography mask in accordance with an embodiment of thepresent invention, wherein a flare sensitive pattern comprises abox-in-box grating structure including a calibration marker in eachgrating, wherein the calibration markers may be used for calibration ofoptical metrology tools;

FIG. 4 shows a more detailed view of a portion of the flare sensitivepattern shown in FIG. 2, showing an enlargement of the calibrationmarkers and the distance to end of the grating lines;

FIG. 5 is a cross-sectional view of the mask shown in FIGS. 3 and 4; and

FIG. 6 is a graph showing an example of the correlation of optical andSEM measured misalignments in accordance with an embodiment of thepresent invention.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the preferredembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely lithography of semiconductordevices. The invention may also be applied, however, to othertechnologies that require lithography in the manufacturing process, forexample.

In U.S. patent application, Ser. No. 10/964,102, filed concurrentlyherewith, entitled “Measuring Flare in Semiconductor Lithography,” whichis hereby incorporated herein by reference, flare is measured byexposing a resist on a semiconductor workpiece using a lithography mask308 having a flare sensitive first test pattern formed thereon, as shownin FIG. 2A, and asymmetrically exposing the resist using a block-shapedopaque second test pattern, using a lithography mask 318 such as the oneshown in FIG. 2B. As described in this related patent application, thefirst test pattern may comprise a plurality of parallel opaque regionsor gratings 314 in an otherwise transparent mask arranged in rows 314 aand 314 b and columns 314 c and 314 d, as shown. If there is flare inthe lithography system, the corner of the resist patterned by the firsttest pattern closest to the opaque second test pattern will be exposedto more stray light due to flare, and will exhibit line shortening ongratings of the first test pattern in that corner on a resist on awafer. The amount of flare is determined by measuring the patternedregion of the test pattern on the resist close to the opaque testpattern, compared to the patterned region of the test pattern fartherfrom the opaque test pattern, in one embodiment. The differencerepresents the amount of flare in the optical system. The effect ofexposure to the two test patterns is that one corner of the gratingsformed on a resist is prone to additional end of line shortening due toflare compared to the opposite corner. An optical alignment measuringtool is used to determine the relative grating widths.

However, in addition to flare, the optical measurement tool itself alsohas diffraction effects. Thus, not all of the line shortening that ismeasured by the optical alignment measuring tool may be due to flare.Therefore, the lithography system needs to be calibrated to SEMmeasurements in order to ensure that the line shortening measured is dueto flare. However, the gratings of the first test pattern are too longto be measured accurately using a SEM, and thus, a method of accuratelycalibrating the optical measurements is needed.

Embodiments of the present invention achieve technical advantages bydisposing calibration markers within the gratings of the first testpattern so that a SEM can measure the absolute distance between thecalibration marker and the end of the line, e.g., the grating. Ifmarkers of a test pattern are large or comprise a large area, thecontrast of the grating of the test pattern that is seen by an opticalmeasurement tool is decreased, which causes an additional diffractionerror. See Ziger et al., “Understanding Optical End of Line Metrology,”which was incorporated previously herein by reference. In embodiments ofthe present invention, the calibration markers are preferablysufficiently small so that the calibration markers negligibly affect thecontract as viewed by an optical metrology tool. Knowing the layout ofthe calibration marker, e.g., by measuring the grating on the maskhaving the flare sensitive test pattern from the calibration marker tothe end of the grating with a SEM, provides the user with the absoluteline shortening, which can then be used to calibrate opticalmeasurements of line shortening made by an optical measurement tool.

FIG. 3 shows an example of an embodiment of the present invention asapplied to the flare box test pattern shown in FIG. 2A. An array ofcalibration markers 470 is placed in each grating pattern 414 a, 414 b,414 c, or 414 d. Preferably, all of the calibration markers 470 are notplaced in the same location in the plurality of gratings 414; otherwise,the contrast seen by an optical measurement tool would be detrimentallyaffected by the calibration markers 470; e.g., the ability to see theedge of the markers 470 would be diminished. The calibration markers 470are preferably placed diagonally within the gratings within a row orcolumn of gratings, as shown, in one embodiment. However, thecalibration markers 470 may be positioned in any number of designs;e.g., any design preferably not having a substantially horizontal orvertical line shape. Preferably, in one embodiment, the calibrationmarkers 470 each comprise a different location within a grating 414 fora single row 414 a or 414 b or column 414 c or 414 d than every othergrating 414 in each row 414 a or 414 b or column 414 c or 414 d.

In the example shown in FIG. 3, the calibration markers 470 compriserectangular contacts comprising transparent regions in each opaquegrating 414. However, the calibration markers 470 may alternativelycomprise other shapes, such as circles, elipses, or trapezoids, asexamples, although the calibration markers 470 may also comprise othershapes. An enlarged view of a portion of the mask 408 is shown in FIG.4. FIG. 5 shows a cross-sectional view of the lithography mask 408 shownin FIGS. 3 and 4. The mask 408 may comprise a substrate 472 comprising atransparent material such as quartz or glass, as examples, and apatterned opaque material 474 such as chrome formed on the substrate.The opaque material 474 is patterned with the gratings 414 havingcalibration markers 470 formed therein, as shown.

The measurement d_(mask) represents the distance from the middle of acalibration marker 470 to an end of a grating 414 on the mask, e.g.,grating line 4 between lines 3 and 5 in FIG. 4. The distance “d_(wafer)”(not shown in the drawings) used herein refers to the distance measuredon a grating in the same position within the structure (e.g., the testpattern) formed on a resist on a wafer, e.g., between the center of acalibration marker 470 and an end of a grating, as shown.

In accordance with embodiments of the present invention, the distancesd_(mask) and d_(wafer) are measured using a SEM, and the values obtainedare compared to determine the amount of line shortening of thelithography system. This information can then be used as a calibrationfactor for an optical measurement tool, which is later used formeasurements of wafers manufactured using the lithography system. Theoptical measurement tool may comprise a commercially available opticalalignment tool, such as the KLA Archer manufactured by KLA Instrumentsin Santa Clara, Calif., as an example, although alternatively, othermeasurement means may be used for the optical measurements describedherein. The measurements made by the optical measurement tool areadjusted by the calibration factor determined in accordance withembodiments of the present invention, giving a more accurate measurementof line shortening.

An example of an embodiment of the present invention will next bedescribed. The distance between the actual end of the line or grating414 on the mask 408 and the center of the marker 470 may be tabulatedversus an arbitrary line or grating number (e.g., 1, 2, 3, etc., shownin FIG. 4). For example, Table 1 tabulates the distances in μm as afunction of the line for a few gratings 414, and shows a compilation ofthe value d_(mask) for the gratings 414 (e.g., lines 3, 4, and 5) shownin FIG. 4.

TABLE 1 Line d_(mask) 3 0.525 4 0.825 5 1.125

This is repeated for all edges in which absolute line shorteningmeasurements are desired, e.g., the row 414 a or 414 b and column 414 cor 414 d that have been exposed to the second test pattern (as shown inFIG. 2B), to determine the effects of flare. In the absence of maskfabrication errors, this tabulation can be done from the mask design,e.g., from the dimensions of the design; however, the tabulation canalso be done using a SEM measurement of the mask 408, in order to removemask fabrication errors. After tabulation, the mask 408 is used topattern or expose a resist on a semiconductor wafer, and the apparentmisalignment can be measured using an optical measuring tool.

For a flare test as described with reference to FIGS. 2A and 2B, thealignment shift or line shortening of patterns formed on a resist isattributable to differences in flare caused by exposure to the secondtest pattern. However, optical measurement of the pattern formed on theresist is not necessarily the actual line shortening, due to diffractioneffects of the optical measurement tool. To obtain the actual lineshortening due to flare, in accordance with an embodiment of theinvention, the user measures one or more differences between the ends ofthe gratings 414 and the center of the marker 470 on an imaged waferwith this pattern. Note that the center of the marker 470 on the wafercan be determined by measuring the width of the marker 470 and offsetting from either edge. The wafer measurements d_(wafer) are thencompared to the tabulated values d_(mask) to determine the difference,which can then be used to determine a calibration factor for lineshortening measurements made by an optical metrology tool.

The calibration markers 470 preferably comprise substantially square orrectangular contacts in one embodiment, as shown in FIGS. 3 and 4.However, the calibration markers 470 may comprise other shapes, and maycomprise any imperfection. The calibration markers 470 may comprise aminimum feature size of the lithography system, for example. Thecalibration markers 470 may comprise a width of about 0.14 μm or less,and preferably comprise a width of about 0.10 to 0.14 μm in oneembodiment. Each calibration marker 470 may straddle two or moregratings 414, as shown. Preferably, the calibration markers 470 aresmall enough that an optical measurement tool cannot detect them.However, the calibration markers 470 are large enough that a SEM candetect them. The calibration markers 470 preferably comprise atransparent region or abruption within an otherwise opaque grating 414,for example, as can be seen in the cross-sectional view shown in FIG. 5.

Next, an example an embodiment of the present invention used tocalibrate the measurement of flare in the flare measurement system asdescribed with reference to FIGS. 2A and 2B, wherein a first testpattern that is sensitive to flare, and a second test pattern that isopaque and asymmetrically positioned with reference to the first testpattern, are used to expose a resist. Assume that under particularconditions, the measured misalignment conditions in the horizontaldirection for three gratings 414 of a mask such as mask 408 shown inFIG. 3, on an optical measuring tool is 0.130 μm, 0.109 μm, and 0.22 μm,as shown in Table 2, below.

TABLE 2 Line d_(mask) d_(wafer left) Left = d_(wafer left−)d_(mask)d_(wafer right) Right = d_(wafer right−dmask) Right − Left Optical 30.525 0.428 −0.097 0.520 −0.005 0.092 4 0.825 0.721 −0.104 0.820 −0.0050.100 5 1.125 1.025 −0.100 1.119 −0.006 0.094 Summary 0.095 0.130 30.525 0.450 −0.075 0.517 −0.008 0.067 4 0.825 0.750 −0.075 0.824 −0.0010.075 5 1.125 1.049 −0.076 1.118 −0.007 0.069 Summary 0.070 0.109 30.525 0.348 −0.177 0.520 −0.005 0.172 4 0.825 0.653 −0.172 0.821 −0.0040.168 5 1.125 0.948 −0.177 1.124 −0.001 0.175 Summary 0.172 0.220SEM measurements are taken of the distance between the end of a gratingline 414 and the center of the calibration marker 470 on both the left(d_(wafer left))and right (d_(wafer right)) sides of the calibrationmarker 470. These measurements are summarized in Table 2 along with therelevant calculations. The dimensions “d_(wafer left)” and“d_(wafer right)” represent SEM measurements. The value in the columnlabeled “optical” in italics indicated the measured opticalmisalignments.

In the first line of Table 2, “Left=d_(wafer left)−d_(mask)” (−0.097)indicates the difference of the SEM measurements of d_(mask)(0.525) andd_(wafer left)(0.450) for grating line 3. Likewise,“Right=d_(wafer right)−d_(mask)” (−0.005) indicates the difference ofthe SEM measurements of d_(mask) (0.520) and d_(wafer right) (0.450) forgrating line 3. “Right−Left” (0.092) indicates the difference of“Right=d_(wafer right)−d_(mask)” (−0.005) and“Left=d_(wafer left)−d_(mask)”(−0.097), which equals 0.092. This processis repeated for one or more gratings 414 in the test pattern. Theresults are averaged, giving the summary value of “Right−Left” of 0.095,as shown in Table 2, which is the average line shortening according tothe SEM results, of the lithography system. The amount of lineshortening measured by an optical measurement or metrology tool was0.130, for the same pattern. The tests may be repeated a number of timesto determine a more accurate calibration factor. The SEM measurementsare preferably performed at a relatively high magnification, to reducemagnification errors, for example.

FIG. 6 shows the correlation between optical and SEM measurements. Thesedata indicate that the true line shortening as measured with a SEM isroughly 86% of that measured optically less a 0.022 constant offset.

The method described herein can be used to calibrate vertical featuressuch as the gratings 414 shown in rows 414 a and 414 b of FIG. 3, andfeatures comprising other pitches, line sizes, and other shapes, asexamples.

In summary, the measurement of line shortening with optical measurementtools has two components; a diffraction component and an actual lineshortening component. To obtain the actual line shortening component, atrue measurement is needed, which is provided by embodiments of thepresent invention, with SEM measurements of portions of the gratingsbetween the calibration marker 470 and the ends of the gratings 414, andcomparing wafer to mask measurements. The difference between the SEMmeasurements of the wafer and the mask is the actual line shorteningcomponent. The difference between the measurements made by an opticalmetrology tool and the SEM measurements (of the calibration markerportion of the gratings 414) is the component of line shorteningmeasurement made by the optical measurement tool that is due todiffraction.

The SEM measurements of the portions of the gratings 414 (e.g., from thecalibration marker 470 to the end of the grating) are at a highresolution and are highly accurate, because the portions measured aresmaller than an entire grating 414, which would be less accurate tomeasure, at a lower resolution. Because the calibration markers arepositioned at different locations within a row or column of gratings414, the optical measurement tool will not “see” or optically detect themarkers. For example, if the calibration markers were placed in the samelocation within each grating, i.e., if the calibration markers 470 wereplaced close to the edge of the gratings 414, the calibration marker 470could be consumed by the line shortening, if there was a large enoughamount of line shortening in the system.

Embodiments of the present invention provide methods of calibratingmeasurements taken with an optical measurement tool. The amount ofmeasurement error due to line shortening is determined using the novelmethod and lithography task described herein, and the measurement erroris used to calibrate measurements taken by an optical measurement tool.More particularly, measurements taken by an optical measurement tool areadjusted by the calibration factor determined herein.

Advantages of embodiments of the invention include providing improvedmethods of calibrating line shortening measurements. An absolutedistance between a calibration marker and the end of a grating line ismeasured on a lithography mask and on an image patterned on a resist,providing an absolute line shortening measurement. The effects ofdiffraction can be distinguished from the effects of flare on lineshortening. The method reduces the amount of error in the measurement ofline shortening.

Although embodiments of the present invention and their advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, it will be readily understood by those skilled in the artthat many of the features, functions, processes, and materials describedherein may be varied while remaining within the scope of the presentinvention. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed, thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

1. A method of calibrating line shortening measurements, the methodcomprising: providing a workpiece, the workpiece comprising a layer ofphotosensitive material disposed thereon; exposing a selected region ofthe layer of photosensitive material to energy through a flare sensitivefirst test pattern and exposing said selected region to energy through asecond test pattern, said first test pattern and said second testpattern disposed on at least one mask, said second test patterncomprising an opaque or attenuated region located proximate a portion ofthe flare sensitive first test pattern, the flare sensitive first testpattern defining a first area having a first and a second dimension andcomprising a plurality of parallel gratings arranged in one of rows andcolumns and each of said plurality of parallel gratings having a lengththe same as one of said first and second dimensions, and wherein acalibration marker is disposed within at least one of the plurality ofparallel gratings; developing the layer of exposed photosensitivematerial; measuring features formed on the layer of photosensitivematerial by portions of the flare sensitive first test pattern that areproximate the second test pattern to determine a first line shorteningmeasurement resulting at least partially because of the close proximityof said opaque or attenuated region of said second test pattern to saidfirst test pattern; determining a second line shortening measurementusing the calibration marker; and comparing the second line shorteningmeasurement to the first line shortening measurement to determine acalibration factor for line shortening measurements.
 2. The methodaccording to claim 1, further comprising calibrating results of thefirst line shortening measurement using the second line shorteningmeasurement.
 3. The method according to claim 1, wherein determining thesecond line shortening measurement comprises: measuring a length of theat least one grating of the first test pattern of the at least one mask;measuring a first side of the at least one grating formed by the atleast one grating of the first test pattern on the layer ofphotosensitive material between a calibration marker and an end of theat least one grating; measuring a second side of the at least onegrating formed by the at least one grating of the first test pattern onthe layer of photosensitive material between the calibration marker andthe other end of the grating comprises using a scanning electronmicroscope (SEM) determining a difference D1 of the first sidemeasurement and the mask measurement; determining a difference D2 of thesecond side measurement and the mask measurement; and determining adifference of D1 and D2 to determine a second line shorteningmeasurement.
 4. The method according to claim 1, wherein exposing thelayer of photosensitive material comprises: providing a first mask, thefirst mask comprising the first test pattern; providing a second mask,the second mask comprising the second test pattern; exposing the layerof photosensitive material to energy through the first mask, forminglatent features on the layer of photosensitive material; aligning thesecond mask asymmetrically to the latent features formed on the layer ofphotosensitive material; and exposing the layer of photosensitivematerial to energy through the second mask.
 5. The method according toclaim 1, wherein exposing the layer of photosensitive material comprisesproviding a single mask.
 6. The method according to claim 5, whereinexposing the layer of photosensitive material comprises providing thesingle mask having a first region comprising at least one first testpattern and a second region comprising at least one second test pattern,further comprising: exposing the layer of photosensitive material toenergy through the first region of the single mask, forming latentfeatures on the layer of photosensitive material; aligning the secondregion of the single mask asymmetrically to the latent features formedon the layer of photosensitive material; and exposing the layer ofphotosensitive material to energy through the second region of thesingle mask.
 7. The method according to claim 6, wherein providing thesingle mask comprises providing a mask comprising a first test patterncomprising a top row of opaque gratings, a bottom row of opaquegratings, a left column of opaque gratings, and a right column of opaquegratings, and wherein the second test pattern comprises an opaque orattenuated pattern larger than the first test pattern.
 8. The methodaccording to claim 7, wherein aligning the second region of the singlemask asymmetrically comprises aligning the single mask closer to onecorner of the plurality of gratings.
 9. The method according to claim 7,wherein providing the single mask comprises providing a mask having twofirst test patterns in the first region, and two second test patterns inthe second region, wherein the first test patterns and the second testpatterns are aligned on a common axis.
 10. The method according to claim7, wherein providing the single mask comprises providing a mask having aplurality of sets of test patterns, each set of test patterns comprisingtwo first test patterns in the first region, and two second testpatterns in the second region, wherein the first test patterns and thesecond test patterns of each set of test patterns are aligned on acommon axis.
 11. The method according to claim 1, wherein determiningthe second line shortening comprises: measuring a length of at least onegrating of the first test pattern of the at least one mask; measuring afirst side of the at least one grating formed by the at least onegrating of the first test pattern on the layer of photosensitivematerial between a calibration marker and an end of the at least onegrating; measuring a second side of the at least one grating formed bythe at least one grating of the first test pattern on the layer ofphotosensitive material between the calibration marker and the other endof the at least one grating; determining a difference D1 of the firstside measurement and the mask measurement; determining a difference D2of the second side measurement and the mask measurement; and determininga difference of D1 and D2 to determine a second line shorteningmeasurement.
 12. The method according to claim 1, further comprising:using the calibration factor to calibrate a lithography system; andusing the lithography system to manufacture a semiconductor device. 13.A method of manufacturing a semiconductor device, the method comprising:providing a test workpiece having a first layer of photosensitivematerial formed thereon; using a lithography system, exposing a selectedregion of the first layer of photosensitive material of the testworkpiece to energy through a flare sensitive first test patterncomprising a plurality of parallel gratings arranged in one of rows andcolumns and defining a first area having a first and a second dimensionand each of said parallel gratings having a length the same as one ofsaid first and second dimensions, and exposing said selected region toenergy through a second test pattern comprising an opaque or lightattenuating region formed on at least one mask and, at least one of theplurality of the parallel gratings comprising a calibration marker;developing the first layer of photosensitive material of the testworkpiece; measuring features formed on the first layer ofphotosensitive material by portions of the flare sensitive first testpattern that are proximate the second test pattern to determine a firstline shortening measurement of the lithography system resulting at leastpartially because of the proximity of said opaque or attenuated regionof said second test pattern to said first test pattern; determining asecond line shortening measurement of the lithography system using thecalibration marker; comparing the second line shortening measurement tothe first line shortening measurement to determine a calibration factorfor line shortening measurements for the lithography system; adjustingresults of the first line shortening measurement by the calibrationfactor; providing a semiconductor device, the semiconductor devicehaving a second layer of photosensitive material formed thereon; usingthe lithography system to expose the second layer of photosensitivematerial; developing the second layer of photosensitive material; andusing the second layer of photosensitive material to pattern a materiallayer of the semiconductor device.
 14. The method according to claim 13,wherein measuring features formed on the first layer of photosensitivematerial to determine the first line shortening measurement comprisesdetermining the amount of flare of the lithography system.
 15. Themethod according to claim 13, wherein determining the calibration factorcomprises determining diffraction effects of the lithography system. 16.The method according to claim 13, wherein the lithography systemincludes at least one lens and at least one other component, wherein ifthe adjusted results of the first line shortening measurement indicatethat an excessive amount of flare is present in the lithography system,further comprising servicing or replacing the at least one lens or theat least one other component of the lithography system.
 17. The methodaccording to claim 13, wherein measuring features formed on the firstlayer of photosensitive material to determine the first line shorteningmeasurement comprises using an optical measurement tool, and whereindetermining the second line shortening measurement comprises using anscanning electron microscope (SEM).
 18. The method according to claim13, wherein determining the second line shortening measurementcomprises: measuring a length of at least one grating of the first testpattern of the at least one lithography mask; measuring a first side ofthe at least one grating formed by the at least one grating of the firsttest pattern on the first layer of photosensitive material between acalibration marker and an end of the at least one grating; measuring asecond side of the at least one grating formed by the at least onegrating of the first test pattern on the first layer of photosensitivematerial between the calibration marker and the other end of the atleast one grating; determining a difference D1 of the first sidemeasurement and the mask measurement; determining a difference D2 of thesecond side measurement and the mask measurement; and determining adifference of D1 and D2.